A common process for producing granules from a liquid composition is described in U.S. Pat. No. 5,779,945. The focus of U.S. Pat. No. 5,779,945 is the treatment and sorting of generated granules with different sizes. Herein a gas/solids separating apparatus such as a cyclone or a scrubber is used to separate solid material from the off-gas stream of the apparatus. Advanced treatment of the off-gas stream is not taken into further account.
In U.S. Pat. No. 4,370,198 the off-gas of the granulation unit is sent to a dust separation cyclone followed by a continuous wet scrubber which both contributes to the scrubbing off said off-gas stream. The scrubbing liquid used is part of the solution or suspension to be proceeded and the scrubbing liquid leaving the wet scrubber is fed back directly into the granulation unit. Exemplarily, the described process can be achieved for the production of sodium chloride, urea, saccharose or ferric oxide, respectively. Hereby the scrubbing liquor is part of the solution or suspension to be processed and is send directly back into the granulation unit. This process can be only achieved for dust scrubbing but is not suitable for ammonia scrubbing.
A further example for an apparatus and a method for wet type simultaneous cleaning and dust-removing gas treatment in a horizontal cross-flow scrubber are disclosed in EP 0853971 A1. This invention performs the removal of pollutants and dust in a packed tower.
In a urea plant used air exiting a urea granulator that is equipped with a fluidized bed contains in addition to urea dust also ammonia. This ammonia contamination needs to be removed before the off-gas stream can be vented into the atmosphere.
Removing ammonia from an off-gas stream is a well-known technology. Usually the off-gas stream is treated with an acidic scrubbing solution. This scrubbing solution can be easily manufactured by adding an acid such as nitric acid or sulphuric acid to water. The ammonia is removed from the gas stream by chemical absorption and converted to the corresponding ammonium salt. The use of nitric acid produces ammonium nitrate (AN), and the use of sulphuric acid produces ammonium sulphate (AS) respectively. These ammonium salt-containing solutions can be used for the production of ammonium sulphate fertilizer or NPK fertilizer, the technology for this is state of the art.
In a urea plant, ammonium salts do not occur in the process and cannot easily be processed at existing urea facilities. A conventional urea production facility therefore has only the following options to reduce gaseous ammonia emissions from the granulation plant:                to discharge the diluted ammonium salt solution to a waste water stream,        to concentrate the diluted ammonium salt solution up to a concentration which can be utilized by other plants, e.g. NPK,        to produce UAS (urea/ammonium sulphate) fertilizer with a high sulphur content,        to produce UAN (urea/ammonium nitrate) solution.        
All of these alternatives require significant investments and changes to operating conditions or entail changes of the product composition and characteristics. All above options result in new products that require additional facilities for transport and handling as well as energy utilities in expensive quantities. As a consequence, nowadays, urea facilities are run without efficient ammonia removal causing severe environmental problems. Therefore, ammonia removal from a urea facility is a challenging task that needs to be solved.
An alternative solution is described in WO 03/099721. This invention relates to a process for removing ammonia from an ammonia-containing gas stream by converting the ammonia in the ammonia-containing gas stream with an organic acid into an ammonium salt, whereas the obtained ammonium salt is contacted, at elevated temperature, with peroxide. The ammonium salt is hereby converted into a NH3, CO2 and H2O containing mixture in a decomposer and can readily be reprocessed in a urea synthesis unit. The peroxide is supplementary to the common process and may relate to other negative accompaniments. Also, for the conversion of the ammonium salt into NH3, CO2 and H2O a separate decomposer in addition to the normal plant layout is required. This emerging gas stream can not be reprocessed in a granulation unit but needs to be recycled in a urea synthesis unit.
Reductions of ammonia emissions are also described in M Potthoff, Nitrogen+Syngas, [online], July. August 2008, pages 39-41. In FIG. 1 a combined dust and acidic scrubber system is shown. The ammonia is absorbed in the acidic scrubbing section and converted into ammonium sulphate. The ammonium sulphate solution is added to the recycle flow going back to the evaporation section. In this unit it is mixed with urea melt from the urea synthesis unit. The concentrated liquor stream from the evaporation is conveyed into the urea granulator. The condensate coming out of the evaporation unit is utilised as makeup for the combined dust/ammonia scrubbing system. With this so called Ammonia Convert Technology ammonia in off-gas can be reduced to 30 mg/Nm3. The technology without acidic scrubbing as shown in Brochure Urea, [online], 12-2007, pages 1-24 reduces ammonia in off-gas only to values of around 160 mg/m3.
The ammonia convert technology described in M Potthoff, Nitrogen+Syngas, [online], July. August 2008, pages 39-41 implicates still several disadvantages. First of all, the water balance in this system is a critical parameter. If disturbed, urea synthesis will be contaminated with ammonium sulphate or alternatively large amounts of waste water need to be treated. In addition, mixing of acidic solution with concentrated urea melt in the evaporation unit has adverse effects on granulation. Moreover, this technology implicates the generation of large amounts of condensate contaminated with ammonium sulphate that needs to be distributed to various scrubbers, including dust and acidic scrubbing technology. Also the remaining ammonia concentration in the off-gas achieved with this technology is still not sufficient or satisfactory for modern urea granulation plants.
In WO 2010/060535 A1 the ammonia convert technology described in M Potthoff, Nitrogen+Syngas, [online], July. August 2008, pages 39-41 is improved in order to achieve ammonia concentrations in off-gas of 10 mg/Nm3. WO 2010/060535 A1 teaches that a scrubber dust stage, that is connected to process coolers, is operated through an ammonium salt solution stream generated in a scrubber acid stage, which is connected to the urea granulator. Therefore the scrubbing system presented in WO 2010/060535 A1 represents an in itself complete closed system as described in the characteristic part of claim 1 of this invention. This technology avoids contamination of the urea melt generated in the urea synthesis unit by building such a complete closed scrubbing system. The disadvantage of this system is that it is very complex in its performance.
In U.S. Pat. No. 5,686,647 a process for preparing urea is described wherein an amount of formaldehyde is added to an off-gas stream containing gaseous ammonia to form hexamethylenetetramine, which is returned into the process before the granulation step. This formaldehyde addition can be performed before or during a washing step with liquid urea solution whereby this washing step serves as dust scrubbing device. The disadvantage of this technology is the relatively high amount of ammonia in the off-gas of circa 90 mg/Nm3 in comparison to the technology presented in WO 2010/060535 A1.